1. Field of the Invention
The present invention relates to a multicast signal transmission power control method that controls a transmission power of a multicast signal that is transmitted by a base station to a plurality of mobile stations through a radio link. Further, the present invention relates to a base station that uses the transmission power control method for the transmission of the multicast signal.
2. Description of the Related Art
Transmission of a multicast signal across a mobile radio link is known. In such multicast communications, a multicast signal is simultaneously transmitted from a base station to each of a plurality of specified mobile stations. Such multicast signals, sent by the base station, contain identical messages and different destinations, and the identical message is delivered to each of the mobile stations specified as the destination stations that receive it.
FIG. 8 shows a conventional multicast signal communication system. In the communication system shown in FIG. 8, a base station 1 is provided, and this base station 1 transmits a multicast signal 4. A plurality of mobile stations 5 are provided in the communication system, and each mobile station 5 receives the multicast signal 4 from the base station 1. For the sake of simplicity of description, only one mobile station 5 is shown in FIG. 8, and the remaining mobile stations 5, which receive the multicast signal 4, are omitted.
In the base station 1, the original message of the multicast signal 4 is supplied from an input terminal 2, and it is input to a transmitter (TX) 3. In the transmitter 3, the carrier wave is modulated in accordance with the original message from the input terminal 2, and the multicast signal 4 is produced. After the modulation is performed, the transmitter 3 transmits the multicast signal 4 to one of the mobile stations 5 based on a predetermined transmission power.
In the mobile station 5 (or one of the plurality of mobile stations 5), the multicast signal 4 from the base station 1 is received at a receiver (RX) 6. After the demodulation of the received multicast signal is performed, the receiver 6 supplies the demodulated multicast signal to an output terminal 7, and it is output from the output terminal 7 to internal units of the mobile station 5.
In the communication system of FIG. 8, the base station 1 is not provided with a mechanism that controls the transmission power of the base station 1 used to transmit the multicast signal 4 to the mobile station 5, such that the transmission power is suited to the received signal quality (or the grade of service) of the mobile station 5. It is difficult for the base station 1 to effectively reduce the transmission power of the base station to the optimum level for transmitting the multicast signal to the mobile stations so as to be in conformity with the received signal quality required for the mobile stations which receive the multicast signal.
In order to ensure that the received multicast signal conforms to the received signal quality of all the mobile stations located with the related cell of the base station 1, it is necessary that the predetermined transmission power, used by the base station 1 of the conventional communication system, be set to a level which is larger than the marginal transmission power needed to transmit the multicast signal from the base station 1 to the mobile stations. The base station 1 must always use the predetermined transmission power that is larger than the smallest possible transmission power, because the base station 1 includes no transmission power control mechanism.
However, when the mobile stations densely exist at locations near the base station 1, the predetermined transmission power of the base station 1 becomes excessively large. This will cause the interference of the transmission power of the base station 1 for the receiver mobile stations which receive the multicast signal with that for the non-receiver mobile stations which do not receive the multicast signal. It is difficult for the conventional multicast signal communication system to provide efficient use of the transmission power of the base station for the transmission of the multicast signal to the mobile stations.